I wouldn't pay the £6. If they don't advertise the charge and you were not made aware of it at the time, then I can't see how you're liable (although I'm no lawyer).
Bristol sent me an invoice for a landing I never made. On my first solo landaway, I arrived, got put in an orbit for 6 orbits, given a clearance to continue to finals, got on base fairly sharply, then told to carry on past as a commercial was up my backside. Then had to orbit again and was told I was number 5!. Not actually needing to land (having proved I could get there), I asked if it would help if I just left. ATC thanked me profusely as they were obviously overworked.
Two days later I got a bill.
One call sorted it out and in the bin it went.
